1. Field of the Invention
The present invention relates to a target of an image pickup tube and a method of producing the same.
More particularly, the present invention relates to transparent conductive electrodes for use in, for example, image pickup tubes of a single tube or a double tube color camera, and to a method of controlling the cross-sectional shape of a transparent conductive film pattern suitable for producing the transparent conductive electrodes.
2. Description of the Prior Art
The signal electrode of an image pickup tube target for use in single or double tube type color cameras is composed of finely striped transparent conductive film. The structure of typical conventional image pickup tube targets having the striped transparent conductive film, as well as the method of producing the same, is as follows.
The color-sensitive target structure in the past is shown in FIG. 1. It is composed of two kinds of the glass substrates 1, 2 on which the tri-color striped filters 3 and the striped electrodes 4 are fixed respectively.
The filter stripes 3 are built in a repeated sequence of red, green, and blue transmission. The electrodes 4 consist of three sets of 216 stripes corresponding to the red, green, and blue filter stripes 3, and are connected by using the multilayered inner connection technique to the common output terminal for that color at both their tops and bottoms. In the drawings reference numeral 9 denotes bus-bars connected to the output terminal.
After polishing on the bottom side of the electrode substrate 7 on which the electrodes are fixed, the filter substrate 8 is cemented to it by means of a resin 5 as shown in FIG. 1. Then the photoconductive material 6 is deposited on the electrode side of the substrate. The target plate is thus completed.
The fundamental structure was concretely devised by Weimer and his co-workers about fifteen years ago.
This method is disclosed, for example, in S. Gray and P. K. Weimer, RCA Review, pp. 413-425, Sept. 1959;
P. K. Weimer, S. Gray et al, IRE Transactions on Electron Devices, pp. 147-153, July, 1960; and
Harold Borkan, RCA Review, pp. 3-16, March, 1960.
Also, the present inventors have reported an article entitled "A Novel Tri-color Pick-up Tube For Use in A Single Tube Color TV Camera", in page 74 of "1974 Iedm Technical Digest".
The electrodes substrate 7 of this type is produced by the method detailed in the specification of U.S. Pat. Ser. No. 3,984,722.
As shown in FIG. 3A, a film 4 of SnO.sub.2 is formed on a glass substrate 2 and a photoresist film is in turn formed on the SnO.sub.2 film 4. The portions of the photoresist film corresponding to a predetermined pattern are exposed and developed in an ordinary manner and the non-exposed portion of the photoresist film is removed to form a mask 21. Thereafter, a sample 17 as shown in FIG. 3A is placed on the target electrode 11 of the RF sputtering apparatus 10 shown in FIG. 2. The internal air is evacuated through the evacuating port 14 so that the pressure inside the apparatus may be below 5.times.10.sup.-6 Torr. Argon gas at a pressure of about 5.times.10.sup.-3 Torr is led into the apparatus through the gas inlet port 13. An RF field is established between the target electrode 11 and the grounded electrode 12 by an RF power source 15 connected through a capacitor 16 between the electrodes 11 and 12. As a result, the argon gas is ionized to bombard the sample 17 so that the SnO.sub.2 film 4 is etched through the mask 21 of the photoresist film due to sputtering phenomenon. The mask 21 is removed, after completion of etching, by rubbing it with the cotton swab in an ordinary photoresist stripper.
According to the method described above, as seen in the sample 18 shown in FIG. 3B, both the SnO.sub.2 film 4 and the photoresist film 21 are etched due to the ion bombardment.
The SnO.sub.2 film stripes 4 of the sample 19 in FIG. 3C, obtained as above, were uniform over the surface of the sample.
As a modification of the above described method, it is possible to use Cr, Ti or Mo film pattern. However, in all of these conventional structures, the angle .theta. formed between the surface of the substrate and the etched portion of the transparent conductive pattern is about 60.degree..
The target section of the image pickup tube is formed by coating the transparent electrode 4 with a photoconductive layer 6, by vacuum evaporation or the like method. FIG. 3D shows an example of this structure in cross-section. It will be seen from FIG. 3D that the portions of the photoconductive layer 6 on the transparent conductive film 4 and the portion of the same directly coating the substrate 2 have different heights from the surface of the substrate 2, i.e. the photoconductive layer is made to spread unevenly. Therefore, electric current is likely to be generated at the edge of this signal electrode, so as to cause an increment of dark current, during functioning of the tube. The increment of the dark current is serious especially in case of the structure incorporating a photoconductive film which exhibits a blocking-contact. Consequently, after a long time operation of the tube, undesirable roughening of the picture surface, as well as after image phenomenon, has been often experienced.
In another example of conventional technique to form a structure as shown in cross-section in FIG. 4, it is attempted to get rid of the disadvantage of the uneven spreading of the photoconductive layer as observed in the foregoing example. More specifically, the spaces between the adjacent islands of the striped transparent conductive film 4 are filled with insulating films 22 such as of glass, to form a smooth surface. However, this technique inconveniently necessitates troublesome steps of process subsequent to the formation of the striped transparent conductive film, such as coating with glass, and polishing and smoothening the glass.